High molecular weight copolymers of perfluoro(methyl vinyl ether) (PMVE) with either vinylidene fluoride (VF2) or tetrafluoroethylene (TFE) are known in the art. Vulcanized elastomer parts having relatively low glass transition temperature (Tg) may be fashioned from such polymers. See for example US 2005/0215741. Such copolymers may be made in the presence of a chain transfer agent of formula RfX, wherein Rf is a perfluorinated alkyl group and X is an iodine or bromine atom, resulting in copolymers having iodine or bromine atom endgroups.